Beyond the conventional use of a single Transmission (Tx) antenna and a single Reception (Rx) antenna, Multiple Input Multiple Output (MIMO) is a communication technology using a plurality of Tx antennas and a plurality of Rx antennas, with the aim to increase the transmission and reception efficiency of data.
When it uses a single antenna, a receiver receives data from a single antenna path. However, when it uses multiple antennas, the receiver receives data from different paths and combines them, thereby achieving whole data. Thus MIMO may increase data rate and transmission capacity and improve the performance of a wireless communication system. In this context, MIMO is considered as a promising future-generation communication technology that can overcome the limit of mobile communication transmission capacity that has been reached due to the recent growth of data communication.
FIG. 1 illustrates channels between multiple Tx antennas and multiple Rx antennas in a MIMO wireless communication system.
Referring to FIG. 1, use of a plurality of Tx antennas and a plurality of Rx antennas at both a transmitter and a receiver increases a channel transmission capacity in proportion to the number of antennas, compared to use of a plurality of antennas at only one of the transmitter and the receiver. Therefore, transmission rate and frequency efficiency are remarkably increased.
The transmission rate increases with the channel transmission capacity. A theoretical transmission rate increase rate Ri may be calculated by [Equation 1].Ri=min(NT,NR)
where NT denotes the number of Tx antennas and NR denotes the number of Rx antennas.
Given a maximum transmission rate Ro that may be achieved in case of a single antenna, a transmission rate R0Ri can be achieved in theory in case of multiple antennas.
For instance, a wireless communication system with four Tx antennas and four Rx antennas may achieve a four-fold increase in transmission rate theoretically, relative to a single-antenna wireless communication system.
Since the theoretical capacity increase of the MIMO wireless communication system was proved, many techniques have been actively studied to increase data rate in real implementation. Some of the techniques have already been reflected in various wireless communication standards including standards for 3rd Generation (3G) mobile communications, future-generation Wireless Local Area Network (WLAN), etc.
Concerning the research trend of MIMO up to now, active studies are underway in many respects of MIMO, inclusive of studies of information theory related to calculation of multi-antenna communication capacity in diverse channel environments and multiple access environments, studies of measuring MIMO radio channels and MIMO modeling, studies of time-space signal processing techniques to increase transmission reliability and transmission rate, etc.
A MIMO wireless communication with NT Tx antennas and NR Rx antennas is mathematically modeled as follows.
Regarding a transmission signal, up to NT pieces of information can be transmitted through the NT Tx antennas, as expressed as the following vector.s=[s1,s2, . . . ,sNT]T  [Equation 2]
A different transmit power may be applied to each piece of transmission information. Let the transmit power levels of the transmission information be denoted by
P1, P2, . . . , PNT,
respectively. Then the transmit power-controlled transmission information vector may be given asŝ=[ŝ1,ŝ2, . . . ,ŝNT]T=[P1s1,P2s2, . . . ,PNTsNT]T  [Equation 3]
The transmit power-controlled transmission information vector may be expressed as a diagonal matrix P of transmit power.
                              s          ^                =                                            [                                                                                          P                      1                                                                                                                                                                                                                                                                                0                                                                                                                                                                                                                P                      2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    ⋱                                                                                                                                                                                          0                                                                                                                                                                                                                                                                                  P                                              N                        T                                                                                                        ]                        ⁡                          [                                                                                          s                      1                                                                                                                                  s                      2                                                                                                            ⋮                                                                                                              s                                              N                        T                                                                                                        ]                                =          Ps                                    [                  Equation          ⁢                                          ⁢          4                ]            
NT transmission signals are generating by multiplying the transmit power-controlled information vector by a weight matrix W. The weight matrix W functions to appropriately distribute the transmission information to the Tx antennas according to transmission channel statuses, etc. The weight matrix W is also called a precoding matrix.
These NT transmitted signals
x1, x2, . . . , xNT 
are represented as a vector x, which may be determined as
                                                        x              =                                                [                                                                                                              x                          1                                                                                                                                                              x                          2                                                                                                                                    ⋮                                                                                                                                      x                          i                                                                                                                                    ⋮                                                                                                                                      x                                                      N                            T                                                                                                                                ]                                =                                                      [                                                                                                                        w                            11                                                                                                                                w                            12                                                                                                    …                                                                                                      w                                                          1                              ⁢                                                              N                                T                                                                                                                                                                                                                                      w                            21                                                                                                                                w                            22                                                                                                    …                                                                                                      w                                                          2                              ⁢                                                              N                                T                                                                                                                                                                                                          ⋮                                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                                                  w                                                          i                              ⁢                                                                                                                          ⁢                              1                                                                                                                                                            w                                                          i                              ⁢                                                                                                                          ⁢                              2                                                                                                                                …                                                                                                      w                                                          i                              ⁢                                                                                                                          ⁢                                                              N                                T                                                                                                                                                                                                          ⋮                                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                                                  w                                                                                          N                                T                                                            ⁢                              1                                                                                                                                                            w                                                                                          N                                T                                                            ⁢                              2                                                                                                                                …                                                                                                      w                                                                                          N                                T                                                            ⁢                                                              N                                T                                                                                                                                                                          ]                                    ⁡                                      [                                                                                                                                                      s                              ^                                                        1                                                                                                                                                                                                          s                              ^                                                        2                                                                                                                                                ⋮                                                                                                                                                                                s                              ^                                                        j                                                                                                                                                ⋮                                                                                                                                                                                s                              ^                                                                                      N                              T                                                                                                                                            ]                                                                                                                          =                                                W                  ⁢                                      s                    ^                                                  =                                  W                  ⁢                                                                          ⁢                  P                  ⁢                                                                          ⁢                  s                                                                                        [                  Equation          ⁢                                          ⁢          5                ]            
where wij denotes a weight for a jth piece of information
ŝj 
transmitted through an ith Tx antenna.
Given NR Rx antennas, signals received at the Rx antennas,
y1, y2, . . . , yNR 
may be represented as the following vector.y=[y1,y2, . . . ,yNR]T  [Equation 6]
When channels are modeled in the MIMO wireless communication system, they may be distinguished according to the indexes of Tx and Rx antennas. A channel between a jth Tx antenna and an ith Rx antenna is represented as hij. These channels between the Tx antennas and the Rx antennas may be expressed as vectors or a matrix by grouping.
FIG. 2 illustrates channels from NT Tx antennas to an ith Rx antenna.
Referring to FIG. 2, the channels from the NT Tx antennas to the ith Rx antenna may be expressed as [Equation 7].hiT=[hi1,hi2, . . . ,hiNT]  [Equation 7]
Hence, all channels from the NT Tx antennas to the NR Rx antennas may be expressed as the following matrix.
                    H        =                              [                                                                                h                    1                    T                                                                                                                    h                    2                    T                                                                                                ⋮                                                                                                  h                    i                    T                                                                                                ⋮                                                                                                  h                                          N                      R                                        T                                                                        ]                    =                      [                                                                                h                    11                                                                                        h                    12                                                                    …                                                                      h                                          1                      ⁢                                              N                        T                                                                                                                                                              h                    21                                                                                        h                    22                                                                    …                                                                      h                                          2                      ⁢                                              N                        T                                                                                                                                          ⋮                                                                                                                                          ⋱                                                                                                                                                                                          h                                          i                      ⁢                                                                                          ⁢                      1                                                                                                            h                                          i                      ⁢                                                                                          ⁢                      2                                                                                        …                                                                      h                                          i                      ⁢                                                                                          ⁢                                              N                        T                                                                                                                                          ⋮                                                                                                                                          ⋱                                                                                                                                                                                          h                                                                  N                        R                                            ⁢                      1                                                                                                            h                                                                  N                        R                                            ⁢                      2                                                                                        …                                                                      h                                                                  N                        R                                            ⁢                                              N                        T                                                                                                                  ]                                              [                  Equation          ⁢                                          ⁢          8                ]            
Actual channels experience the above channel matrix H and then are added with
Additive White Gaussian Noise (AWGN). The AWGN
n1, n2, . . . , nNR 
added to the NR Rx antennas is given as the following vector.n=[n1,n2, . . . ,nNR]T  [Equation 9]
From the above modeled equations, the received signal vector is given as
                                                        y              =                                                [                                                                                                              y                          1                                                                                                                                                              y                          2                                                                                                                                    ⋮                                                                                                                                      y                          i                                                                                                                                    ⋮                                                                                                                                      y                                                      N                            R                                                                                                                                ]                                =                                                                            [                                                                                                                                  h                              11                                                                                                                                          h                              12                                                                                                            …                                                                                                              h                                                              1                                ⁢                                                                  N                                  T                                                                                                                                                                                                                                                        h                              21                                                                                                                                          h                              22                                                                                                            …                                                                                                              h                                                              2                                ⁢                                                                  H                                  T                                                                                                                                                                                                                          ⋮                                                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                                                                        h                                                              i                                ⁢                                                                                                                                  ⁢                                1                                                                                                                                                                        h                                                              i                                ⁢                                                                                                                                  ⁢                                2                                                                                                                                          …                                                                                                              h                                                              i                                ⁢                                                                                                                                  ⁢                                                                  N                                  T                                                                                                                                                                                                                          ⋮                                                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                                                                        h                                                                                                N                                  R                                                                ⁢                                1                                                                                                                                                                        h                                                                                                N                                  R                                                                ⁢                                2                                                                                                                                          …                                                                                                              h                                                                                                N                                  R                                                                ⁢                                                                  N                                  T                                                                                                                                                                                        ]                                        ⁡                                          [                                                                                                                                  x                              1                                                                                                                                                                                          x                              2                                                                                                                                                            ⋮                                                                                                                                                              x                              j                                                                                                                                                            ⋮                                                                                                                                                              x                                                              N                                T                                                                                                                                                        ]                                                        +                                      [                                                                                                                        n                            1                                                                                                                                                                            n                            2                                                                                                                                                ⋮                                                                                                                                                  n                            i                                                                                                                                                ⋮                                                                                                                                                  n                                                          N                              R                                                                                                                                            ]                                                                                                                          =                                                H                  ⁢                                                                          ⁢                  x                                +                n                                                                        [                  Equation          ⁢                                          ⁢          10                ]            
Now a description will be made of Reference Signals (RSs).
In the wireless communication system, a packet is transmitted on a radio channel. In view of the nature of the radio channel, the packet may be distorted during the transmission. To receive the signal successfully, the receiver should compensate for the distortion in the received signal using channel information. Generally, to enable the receiver to acquire the channel information, the transmitter transmits a signal known to both the transmitter and the receiver and the receiver acquires knowledge of channel information based on the distortion of the signal received on the radio channel. This signal is called a pilot signal or an RS.
In case of data transmission and reception through multiple antennas, knowledge of channel states between Tx antennas and Rx antennas is required for successful signal reception. Accordingly, an RS should exist for each Tx antenna.
According to the purposes that they serve, RSs are categorized into four types as listed in Table 1, Channel Quality Indicator-Common Reference Signal (CQI-CRS), Demodulation-Common Reference Signal (DM-CRS), Non-precoded Demodulation-Dedicated Reference Signal (NDM-DRS), and Precoded Demodulation-Dedicated Reference Signal (PDM-DRS). As their appellations imply, a CRS is an RS common to all UEs and a DRS is an RS dedicated to a specific UE. NDM-DRS and PDM-DRS are collectively referred to as “DM-DRS”.
TABLE 1RSTypesNotesCQI-A common reference signal used for channel measurement. TheCRSUE determines a CQI, a Precoding Matrix Indicator (PMI), anda Rank Indicator (RI) based on CQI-CRSs. Thus, preferably,CQI-CRSs are uniformly distributed across a total frequencyband.DM-Although a DM-CRS is a common reference signal used for de-CRSmodulation, it can be also used for channel measurement.Because a plurality of UEs use DM-CRSs for channel mea-surement, a precoding scheme for a specific UE cannot beapplied to the DM-CRSs. Thus, when a transmitter precodes aPhysical Downlink Shared CHannel (PDSCH), it needs tosignal a used precoding matrix on a Physical DownlinkControl CHannel (PDCCH) to a receiver.NDM-A non-precoded dedicated reference signal used for de-DRSmodulation.PDM-A precoded dedicated reference signal used for modulation.DRSThe same precoding scheme is applied to the PDM-DRS andthe PDSCH. Thus there is no need for signaling a usedcodebook on the PDCCH.
An RS is transmitted in a specific time-frequency area of a subframe. With reference to FIGS. 3(a) and 3(b), conventional RS allocation structures will be described below.
FIG. 3(a) illustrates a conventional RS allocation structure in a subframe in a wireless communication system using a normal Cyclic Prefix (CP), and FIG. 3(b) illustrates a conventional RS allocation structure in a subframe in a wireless communication system using an extended CP.
Both the conventional RS allocation structures illustrated in FIGS. 3(a) and 3(b) are designed for four Tx antenna ports. In FIGS. 3(a) and 3(b), the horizontal axis represents time and the vertical axis represents frequency. RSs allocated to the subframes are all CRSs for use in channel measurement and demodulation. Referring to FIGS. 3(a) and 3(b), one subframe includes two slots. Each slot has seven Orthogonal Frequency Division Multiplexing (OFDM) symbols in the wireless communication system using the normal CP, whereas each slot includes six OFDM symbols in the wireless communication system using the extended CP. The extended CP is used generally under a long-delay environment.
In the illustrated cases of FIGS. 3(a) and 3(b), RSs are allocated on a Resource Block (RB) basis. An RB is a set of resource elements each being a resource allocation unit having one subcarrier by one OFDM symbol.
In FIGS. 3(a) and 3(b), reference characters 1, 2, 3 and 4 denote resource elements to which RSs for first to fourth Tx antenna ports are allocated, respectively.
The conventional wireless communications support four Tx antenna ports. Therefore,
RSs for the four Tx antenna ports are allocated in the conventional RS allocation structures of FIGS. 3(a) and 3(b). However, in anticipation of a future wireless communication system adopting more Tx antenna ports, there is a need for a method for transmitting RSs for five or more Tx antenna ports.